This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-267678, filed Sep. 4, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a power supply circuit in which value of output voltage is adjusted in response to control signals, particularly to a power supply circuit which preferably includes a semiconductor integrated circuit.
2. Description of the Related Art
For a power supply circuit included in a semiconductor integrated circuit, particularly in a semiconductor memory device, a set voltage is diversified. Particularly in the power supply circuit for use in a dynamic memory or ferroelectric memory, it is necessary to output voltage which has various values between an external power supply voltage and ground voltage. Additionally, for the voltage to be outputted from the power supply circuit, an optimum set value sometimes differs with characteristics of a processed memory cell. Therefore, the value of the output voltage of the power supply circuit is adjusted in an operation test after the processing.
As a conventional power supply circuit whose output voltage can be adjusted, for example, a circuit shown in FIG. 3 of U.S. Pat. Ser. No. 6,061,289 is known. This power supply circuit is constituted of a ladder circuit including a plurality of resistances, and a two-systems feedback circuit using an operational amplifier.
By characteristics of the feedback circuit using the operational amplifier, the power supply circuit is controlled so that a voltage in a non-reverse input terminal of one operational amplifier OPA is equal to a reference voltage VR supplied to a reverse input terminal and the voltage of the non-reverse input terminal of the other operational amplifier OPB is also equal to the reference voltage VR supplied to the reverse input terminal. Moreover, by the characteristics of the ladder circuit, a total of a first current flowing through a node of a ground voltage VSS from a first node X and a second current flowing through the node of the ground voltage VSS from a second node Y indicates a constant value regardless of set states of control signals A1 to A5.
Moreover, a distribution of first and second currents is changed based on the control signals A1 to A5, thereby the value of a current flowing through a resistance RL connected between the node of the output voltage Vout and the non-reverse input terminal of one operational amplifier OPA is changed, and the value of the output voltage Vout is adjusted.
For example, when the control signals are of 5 bits, the current distribution can be changed in 25=32 stages, and 32 voltages can be set as the output voltages Vout.
However, the conventional power supply circuit uses many resistances, and therefore there is a problem that a chip area increases.
Moreover, since two feedback circuits are used, the power supply circuit is weak at a dispersion in manufacturing a device, and there is a problem in stability of a circuit operation.
As described above, many resistances are used in the conventional power supply circuit, and therefore there is a problem that chip and circuit areas increase. Moreover, since a plurality of feedback circuits are used, the power supply circuit is weak at the dispersion in manufacturing the device, and there is a problem in the stability of the circuit operation.
Therefore, there has been a demand for a power supply circuit in which a chip area does not increase and a steady circuit operation can be achieved.
According to a first aspect of the present invention, there is provided a power supply circuit comprises: a transistor which includes a current path including one end and the other end, and a gate and in which one end of the current path is connected to a supply node of a first voltage and the other end of the current path is connected to a voltage output node; a variable resistance circuit which includes one end, the other end, and a plurality of first resistances and in which one end is connected to the voltage output node, the plurality of first resistances are selected in response to control signals, the selected first resistances are connected in series between one end and the other end, and unselected first resistances are connected to the supply node of a second voltage so as to change a resistance value between one end and the other end; a second resistance connected between the other end of the variable resistance circuit and the supply node of the second voltage; and a comparison circuit which compares the voltage of the other end of the variable resistance circuit with a reference voltage and feeds a signal indicating a comparison result back to the gate of the transistor. According to a second aspect of the present invention, there is provided a power supply circuit comprises: a first transistor with a first polarity, which includes a first current path including one end and the other end, and a gate and in which one end of the first current path is connected to a supply node of a first voltage; a second transistor with a second polarity, which includes a second current path including one end and the other end, and a gate and in which one end of the second current path and the gate are connected to the other end of the first current path; a variable resistance circuit which includes one end, the other end, and a plurality of first resistances and in which one end is connected to the other end of the second current path, the plurality of first resistances are selected in response to control signals, the selected first resistances are connected in series between one end and the other end, and unselected first resistances are connected to the supply node of a second voltage so as to change a resistance value between one end and the other end in response to the control signals; a second resistance connected between the other end of the variable resistance circuit and the supply node of the second voltage; a comparison circuit which compares the voltage of the other end of the variable resistance circuit with a reference voltage and feeds a signal indicating a comparison result back to the gate of the first transistor; and a third transistor with the second polarity, which includes a third current path and gate, whose gate is connected to the gate of the second transistor and whose current path is connected between the supply node of a third voltage and a voltage output node.